Perhaps the most common valves used in the petrochemical, refining, and other industries are gate valves. Major components of gate valves include a body, a valve stem, a sealing member, and seat rings which are engaged by the sealing member to close the valve. Because the valve stem must extend from inside the valve body to outside the body, a stem packing must be installed to effectively seal between the body and the valve stem, thereby containing the product inside the piping system of which the valve is a part. To achieve the necessary dynamic seal to both contain the product within the piping system and allow the valve stem to actuate up and down, the area of the valve stem which makes contact with the stem packing must be smooth, round and have no taper on the outside surface of the valve stem.
When the valve is new, the foregoing conditions are met and product inside the piping system is prevented from leaking to the outside. As the valve ages, chemical corrosion from the product, galvanic corrosion from the packing, and erosion and corrosion from atmospheric conditions all combine to reduce the smoothness of the valve stem that is necessary to maintain the seal. Also, roughness of the valve stem can occur as the valve stem is pulled through the stem packing during normal cycling due to abrasive materials accumulating between the valve stem and the seal. Any of these factors, and particularly combinations thereof, can result in leak paths being created and leakage to occur. This is unsatisfactory for numerous reasons, the most important of which involves increasing enforcement of the federal Clean Air Act which makes prevention of leakage or "fugitive emissions" essential.
Previous attempts at solving the foregoing problems have included the application of the material sold under the trademark "HASTALLOY".RTM.. Such material is partially effective in slowing valve stem deterioration due to chemical attack but is ineffective as to mechanical degradation of the valve stem surface.
While the prior art discloses a variety of valves, no prior disclosure shows a device where leakage is prevented by way of an overlay on the valve stem in the area of the stem packing. The improved valve stem of the present invention departs from the design of prior valve stems, and in doing so, provides a valve which prevents leakage and fugitive emissions as the valve ages.